This is covered in our Surfboard Building Guide. The calculations based on your board's thickness in the midpoint, nose, and tail... the goal is to keep the railbands smooth and flowing so don't get caught up in the numbers too much.
Here is an illustration on railbands but here it is for reference
Here are suggested rail band dimensions.
The railbands will taper in the nose and tail areas up to a 1/2" difference from the midpoint dims. It's all relative to the foil of the board. If you are unsure what the railbands will look like simply mark the railband dimensions at the midpoint, 12" from the nose and 12" from the tail and connect the dots with a pencil. Follow the flow of the foil with the line. If the line looks good and matches the flow of the foil then go cut it. If it doesn't look good make a new line until you find what looks right to you.